Add more useful info in FITS file created by stx_map2fits #222
Description
Some useful information that is often required could be written to the header of the FITS files generated with stx_map2fits.
This includes:
- total number of counts used for imaging
- number of counts in the background (when it has been subtracted, zero otherwise)
- roll angle of Solar Orbiter at the time of observation (this is different than the roll angle in the map exported to FITS, which is usually zero because the data is "de-rotated" by the imaging algorithm)